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Update on feline asthma (Proceedings)

Feline bronchopulmonary disease (FBPD), often referred to as "feline asthma" actually encompasses a group of common, but poorly understood, airway diseases. It is estimated that bronchopulmonary disease affects 1% of the general cat population and > 5% of the Siamese breed. Cats of any age can be affected and there is no clear gender predisposition.

Pathophysiology

Airways have a limited number of ways of responding to inhaled irritants or immunologic stimuli. The airway walls usually become thickened and edematous. These thickened airways then experience a greater degree of narrowing in response to a given amount of smooth muscle contraction, resulting in a smaller luminal diameter. Submucosal glands may become hyperplastic and secrete excessive amounts of thick mucus. These physiologic changes result in narrowed airways and increased airway resistance. Initially, reversible airflow obstruction may be seen (which can respond to medications); however, with time, airway remodeling may occur, resulting in a "fixed" airway obstruction.

In humans, hyperreactivity of the airways is a hallmark of clinical asthma; this has been documented in some cats. True asthma results from an IgE mediated hypersensitivity to airborne allergens. Helper T cells produce a variety of interleukins and direct the release of inflammatory mediators. In humans, asthma often has a genetic component.

Infection with Mycoplasma may also have a role in the generation of feline bronchopulmonary disease. Mycoplasma is an extracellular pathogen which attaches to and destroys ciliated epithelial cells. Mycoplasma infection has been shown to produce airway inflammation and hyperreactivity in mice. In humans, infection with M. pneumoniae has been found to be significantly associated with asthma. Studies have shown that 50% of children experiencing their first asthmatic attack were also infected with M. pneumoniae. Furthermore, those children suffering from mycoplasmal infections were far more likely to suffer from recurrences of asthma. In cats, approximately 25% of feline patients evaluated for bronchopulomonary disease culture positive for Mycoplasma.

Heartworm infection is being increasingly recognized as a cause of bronchial disease in cats. Cats are inherently more resistant to heartworm (HW) infection and tend to have smaller, less frequent infections. However, in ~50% of cats who are exposed to HW but who are able to reject the infection still develop respiratory disease. Exposure to heartworm, even in cats who never are truly infected with adult worms, can still result in proliferative changes in the pulmonary arteries and arterioles, bronchioles and lung parenchyma, leading to "heartworm-associated respiratory disease (HARD)". Potentially up to three-quarters of cats with adult HW infection develop clinical signs.

Another potential cause of feline bronchopulmonary disease is the use of bromide to treat seizures. Two separate studies found that approximately one-third of feline patients treated with bromide began coughing and showing evidence of lower airway disease. In some of these cats, the bronchopulmonary changes were so severe as to result in euthanasia. In cats receiving bromide, eosinophilia was found in the bronchoalveolar fluid.

Clinical Signs

Affected cats typically have intermittent respiratory distress and may be without clinical signs between episodes. In some cats, FBPD can result in severe, potentially fatal respiratory distress. With time, progressive airway changes can develop that result in more severe clinical disease and diminished quality of life. Cats typically display coughing and wheezing. Owners may mistake coughing for gagging or vomiting. An increase in respiratory rate and/or effort is usually present during episodes. Signs may progress to more overt dyspnea wherein the cat may become cyanotic and display open-mouth breathing.

During the physical examination, harsh lung sounds, crackles or wheezes may be ausculted. The clinician may note a prolonged expiratory phase to respiration. An abdominal effort may be noted during respiration. During more severe episodes, the cat may be cyanotic and open-mouth breathing.